There are available ranging system such as that used in Polaroid cameras. These are accurate and dependable devices but are not suitable for close ranging, e.g. distances of two or three inches, although they are dependable for distances of nine inches or more. In many applications that is adequate, but not in mobile robots, for robots often must pass through doorways with only two or three inches of clearance on either side. Such close ranging is also required when mobile robots are operating in obstacle avoidance modes since there may be little room to carry out the avoidance path. Further, avoidance maneuvers should minimize diversion from the designated path. One reason close ranging is not obtained with present ultrasonic systems is that at close range the output pulse is so long that it overlaps the returning echo pulse. Even if the output pulse were shortened, the output pulse would still overlap the returning echo pulse because of the ringing following the end of the output pulse. In one device the echo signal generated by the returning echo pulse is in the neighborhood of 100 millivolts. But the bias on the circuit necessary to set the transducer to respond to the echo pulse is approximately 150 volts, 300 volts peak to peak. Thus even the smallest ringing will overpower the echo signal. In fact, a dual-diode clamping circuit is used to pull the 150 volts down to the breakdown voltage of the diode, i.e. 0.7 volts. But that is 700 millivolts, more than enough to overpower the 100 millivolt echo signal. Present systems require 50 microseconds to bring the ringing voltage from 300 volts peak to peak to 0.7 volts, and an additional 500-600 microseconds to bring it to the one millivolt range.